This invention relates to an electron gun with one directly heatable initial cathode mounted behind an indirectly heatable second blocking cathode which, across several offset supporting braces, is connected with a centrally drilled discoid component of a focusing electrode.
Electron guns of this type are used as sources of high energy electron beams such as those required for vacuum fusion and/or volatilization of materials. For such purposes the blocking cathode consisting essentially of a solid circular disk is generally heated up by a directly heated spirally coiled-wire/cathode which then in its turn emits a focused beam of electrons based on an accelerator potential existing within the system.
U.S. Pat. No. 3,273,003 discloses various forms of a known indirectly heatable blocking cathode, but indicates that the loose ends of the offset supporting braces are connected by a conventional retaining device. The prior art teaches beveling the loose ends of the offset supporting braces and introducing them under grid potential into paraxial clearance holes of a discoid component. This discoid component is coupled by rivets to the front flange of a tubular element. The discoid and the tubular parts thus together constitute a focusing electrode, also frequently termed a Wehnelt cylinder.
The known design, however, entailed the following problems : Beveling of the supporting braces was comparatively difficult and could not always be effected with the required precision and even the non-positive mounting in the paraxial clearance holes led only, after protracted laborious adjustment, to a somewhat approximate coaxial alignment to a central bore in the discoid component. Coaxial alignment is, nevertheless, an important matter since the specific beam: formation is determined by the geometric shape of the center bore. The riveting described above caused an aberration in the surface of the focusing electrode that led to objectionable peak voltages with locally excessive field strengths. Occasionally also, the varying lengths of beveled ends of supporting braces that project from the front of the focusing electrode resulted in additional peak voltages. These peak voltages promoted flashovers which must be avoided as far as possible in vacuum plants.
Also, the blocking cathode as well as the discoid component of the focusing electrode are exposed to a not insignificant amount of wear and tear conditioned by the charge and, further, if the electron gun has to be retooled, the blocking cathode and the discoid component must be replaced. In both cases the rivets previously mentioned must be reamed out. This drilling process is permissibile only a few times before the parts in question become unserviceable.